Field of the Invention
Embodiments of the invention relate to polymers that are conductors and to lithium-ion batteries made from those conductors. In typical embodiments the polymers are used as single-ion conductors.
Background of the Related Art
For some time, lithium-ion batteries have been the technology of choice as rechargeable energy storage devices for portable electronics and electric vehicles. Electrolytes that conduct electricity by ions between electrodes constitute an integral part in lithium-ion batteries. Single-ion conducting electrolytes in which the anionic species is anchored to a polymer and becomes immobile present a unique alternative to traditional binary salt conductors.
Theoretically, single-ion electrolytes with a lithium-ion transference number (tLi+) of unity can eliminate the salt concentration gradient and polarization loss in the cell that develops in a binary salt system. This results in substantial improvements in materials utilization for high power and energy densities. Traditionally the best single-ion electrolytes have ambient temperature Li+ conductivities in the range of 10−7˜10−5 S cm−1, which are at least two orders of magnitude smaller for practical applications.
Nearly all of liquid and polymer electrolytes that currently prevail in both prototype and commercial lithium-ion batteries have been binary salt conductors where both lithium ions and their counter-anions migrate between electrodes during charging and discharging process. The conductivity of binary salt conductors is actually dominated by the motion of anions, as anions of salt have very high mobility and move 5-10× faster than Li+ regardless of the choice of anion. For example, polymer electrolytes composed of the Li salts (e.g. LiXF6, X=P, As, Sb) dissolved in coordination polymers, such as poly(ethylene oxide) (PEO), typically have a value of tLi+ between 0.2 and 0.3, i.e., only 20-30% of the measured conductivity is associated with Li+ mobility.
There is, however, no electrode reaction for the anions. As a result, the buildup of the anions at the electrode/electrolyte interface causes concentration polarization, leading to loss of power drawn from the battery. Hence, the free movement of anions needs to be limited or totally eliminated, which has been realized by covalent attachment of the anions to the polymer backbones to form single ion conductors (i.e. ionomers).
Due to the size and relatively immobile nature of the polymer chains, only cations are able to migrate over long distances in the solid state on reasonable time scales, and a unity tLi+ can be achieved in single-ion conductors. The advantages of the employment of single-ion conductors in batteries have been long recognized, including a spatially uniform anion distribution that enables the passage of larger currents through the cell, and lower joule heat per unit of current that lessens the chance of thermal runaway, and the absence of electrochemical interactions of anions with electrodes for improved stability.
Several classes of single-ion conductors have been reported in the past, however with modest success, as this approach significantly depresses the overall electrolyte conductivity. It has been widely accepted that ion conduction in polymer electrolytes is strongly correlated with the local segmental motion and thus with the glass transition phenomena of the polymers.
Consequently, the known single-ion conductors are mainly based on low-glass transition temperature (Tg) polymers such as PEO and polysiloxane. These approaches so far only resulted in limited improvement in room-temperature ionic conductivity. Moreover, the utilization of the low-Tg polymers may scarify the mechanical integrity and thermal stability of the membranes, which is an additional hurdle for the single-ion conductors as they are also often expected to play the role of separators between the electrodes.